The ADC field includes a variety of different types of ADC data carriers and ADC readers operable to read data encoded in such data carriers. For example, machine-readable symbols readers may be used to read information encoded in the optical pattern of machine-readable symbols, such as one- or two-dimensional symbols (e.g., barcode symbols). Wireless radio or microwave frequency interrogators, for example RFID readers, may be used to read information from and/or write information to transponders, for example RFID transponders, commonly referred to as RFID tags.
RFID transponders or tags may store data in a wirelessly accessible memory, and may include a discrete power source (i.e., an active RFID tag), or may rely on power derived from an interrogation signal (i.e., a passive RFID tag). RFID readers typically emit a wireless interrogation or inquiry signal that causes the RFID transponder to respond with a return wireless signal encoding the data stored in the memory. The wireless signals typically have wavelengths falling in the radio or microwave portions of the electromagnetic spectrum. Whether radio or microwave frequencies are employed, such signals are commonly referred to as RF signals. Such a convention is adopted herein and throughout the attached claims.
Identification of an RFID transponder or tag generally depends on RF energy produced by a reader or interrogator arriving at the RFID transponder and returning to the reader. Multiple protocols exist for use with RFID transponders. These protocols may specify, among other things, particular frequency ranges, frequency channels, modulation schemes, security schemes, and/or data formats.
RFID transponders typically include a semiconductor device (e.g., a chip) and one or more conductive traces that form an antenna. The semiconductor device includes an integrated circuit that typically includes memory, logic circuitry and power circuitry. Typically, RFID transponders provide information stored in the memory in response to the RF interrogation signal received at the antenna from the interrogator or reader. Some RFID transponders include security measures, such as passwords and/or encryption. Many RFID transponders also permit information to be written or stored in the memory via an RF signal.
In many environments there may be multiple transponders within range or in the field of an interrogator or reader. Where transponders identifiers are not known beforehand, responses to interrogation signals from the various transponders may collide, making it difficult or even impossible to recover information encoded in the responses. To address such, many interrogation systems employ various methods to singulate transponders.
A common approach to singulation employs the Q-algorithm specified under the Gen2 protocol. The Q-algorithm is similar to slotted Aloha algorithm. In particular, under the Q-algorithm the interrogator or reader determines a value for a query or Q-value, and wirelessly instructions all transponders to respectively randomly generate two numbers, a first number and a second number. For example, the first number is randomly generated to have a value between 0 and 2Q-1 and is used to set a counter in the transponder. The second number is randomly generated to have, for example, a value between 0 and 216. The interrogator or reader then transmits a signal that instructs each transponder to decrement the counter by one, and to respond to the interrogation or inquiry only if the resulting value of the transponder's respective counter is zero. If no transponders respond, the interrogator or reader transmits a signal instructing all transponders to decrement their respective counters. If two or more transponders respond, the interrogator or reader transmits a signal instructing the responding transponders to wait for a next cycle, while instructing the other non-responding transponders to decrement their counters. If only one transponder responds, the interrogator or reader sends a signal instructing the responding transponder to transmit the second number. The interrogator or reader acknowledges receipt of the response signal encoding the second number, and causes the responding transponder to temporarily stop responding (i.e., nap) to interrogations or inquiries.
Current approaches to selecting the value for query or Q-value are typically “blind” adaptive approaches, such as those described in Maguire, Y., Pappu, R., An Optimal Q-Algorithm For The ISO 1800-6C RFID Protocol, IEEE Transactions on Automation Science and Engineering, Vol. 6, Issue 1, January 2009, pages 16-24; and U.S. patent application publication Serial No. 2005-0280505. Such approaches make an initial guess at setting the value of the query or Q-value, then refine the value of Q over repeated cycles. Readers cannot decode and hence discard collided responses where two or more transponders respond concurrently. Conventional understanding is that such collisions or superposition of responses is useless. Under conventional approaches collisions require another round of communications, and hence lost time. Such is a particular detriment to successful interrogation of a crowded field.
New approaches for rapidly and accurately estimating or determining the total number of transponders in a field are desirable.